Needs polish.
The game concept is simple and fun, but it needs a bit of fixing up.
The graphics were a bit too low-resolution. If you're to make a game like this, try to make sure that you're using high quality images. It's ultimately better to draw your own graphics if you're good at it, but otherwise try to find good images on the internet.
The gun sound effects were too loud throughout. Try and tone those down and make sure that their volumes are scaled from the real ones (if that makes any sense). Another thing you could have done here is add some ambient war music in the background.
Gameplay was a bit glitchy, and often I found myself going on to the next level without killing all of the enemies on the level before. In some of the levels I was faced with invincible targets that I couldn't kill nor could I kill them, resulting in me Right-click > Forwarding into the next levels to complete the game. One last point here is that there is no indicator of how close you are to dying, and it was a bit weird to be shooting stuff and then all of a sudden dying.
In controls, most of them were fine, but the sniper was just plain awkward. Snipers aren't that shaky unless you're trying to snipe from a helicopter piloted by a rookie without a tripod. Lessen the amount of shake on the reticle, and it should be fine. The sniper should also be able to zoom, considering it's a long-distance weapon.
The plot is a major problem; it's pretty vague as to what you're doing and even more vague as to why. For your next game, try to write a better story. It really does help games out.
This type of game has potential, so fix the problems and it will definitely turn out good.